The Cantonese learned in schools is not the colloquial (spoken) Cantonese so yes, they will use what you called the "Mandarin grammar", or standard. Most Cantonese songs use the standard rather than the colloquial, because a) it's easier to understand, b) a lot of the spoken Cantonese words do not have written equivalents, and c) it sounds better. Some artists use the colloquial, but they may use a slang which would be hard to understand for those who don't know it. So yes, ta will be used instead of kui, si instead of hai, etc..